Citrus is one of the most important fruit crops grown worldwide, predominantly in tropical and subtropical areas. According to FAO, in 2019, nine countries produced 70.23% of the total citrus production worldwide, and eight countries produced 71.18% of the total lemon production worldwide, with Argentina in the fourth place producing 9.28% (Table 1) (FAO, 2021). Regarding lemon industrialization, Argentina is the most important country worldwide, ranking first in exporting essential oil and lemon juice. In Argentina, Tucumán leads the production of fresh lemon fruit, with 1.3 million tonnes, representing 77.57% of the total production and producing 90% of the total essential oil (Ministerio de Agricultura, Ganadería y Pesca de la Nación Argentina, 2022).
Table 1
Values of total production correspond to data from 2019.
Ranking | Country | Total Production (%) |
| | Citrus | Lemon |
1 | China | 26.25 | 12.99 |
2 | India | 9.26 | 16.96 |
3 | Brazil | 13.67 | 7.36 |
4 | Mexico | 5.85 | 13.16 |
5 | Spain | 4.18 | 4.31 |
6 | Egypt | 3.22 | - |
7 | Turkey | 2.99 | 4.63 |
8 | Argentina | 2.41 | 9.28 |
9 | Iran | 2.40 | - |
10 | South Africa | - | 2.49 |
TOTAL (%) | | 70.23 | 71.18 |
TOTAL (miles tn) | | 143,755.6 | 20,529.6 |
Citrus production is constantly threatened by pathogens that cause considerable economic losses and severe social impacts. Among the major diseases is citrus canker (CCK), caused by the Gram-negative bacteria Xanthomonas citri subsp. citri (Xcc) (Vallejo et al., 2019). The causal agent is present in all citrus-growing areas worldwide and can infect during all seasons (Goto, 1992). It affects plant growth, fruit quality, and yield, and thus, endangers the healthy progress of the citrus industry. X. citri pathogenicity is a complex process that involves bacterial attachment to the surface, microcolony formation, and biofilm development on citrus surfaces (Graham et al., 2000; Rigano et al., 2007; Vojnov & Marano, 2015). The bacterial communication is denominated Quorum sensing and plays an essential role in bacterial adherence to the plant surface and its virulence (Siciliano et al., 2006). After this adherence, X. citri can colonize the mesophyll through natural openings in young susceptible tissues or wounds in older tissues (Lujan et al., 2019; Martins et al., 2020).
Nowadays, copper-containing products are the most used agrochemicals to control canker disease (Das, 2003). However, their main problem is their limited persistence over time, with the consequent need for several applications to ensure efficiency (Behlau et al., 2012; Sena Vélez, 2015; Machado et al., 2021). Moreover, this continuous practice negatively impacts the environment, contaminates water bodies, accumulates in the soil, and may also have a toxic effect on healthy plants and microorganisms (Behlau et al., 2010). Furthermore, prolonged treatment with such products may cause bacterial resistance (Marco and Stall, 1983; Sena Vélez, 2015; Behlau et al., 2011), making it even more challenging to eradicate the disease. For instance, copper resistance within the X. citri population was demonstrated (Canteros, 1999).
Biological control strategies have emerged as healthier and more sustainable alternatives to agrochemicals for the control of numerous plant diseases (Bayer Crop Science, 2020), including citrus canker (Rodrigues Marin et al., 2019; Villamizar and Caicedo, 2019; Poveda et al., 2021; Villamizar et al., 2020). For example, diverse biological control agents were reported to control citrus canker through different mechanisms, such as disruption of X. citri Quorum sensing compromising the biofilm formation and canker development, as well as antibiosis, induction of plant defence responses, space competition, cell lysis, and plant growth promotion (Poveda et al., 2021). In this context, agro-industrial wastes in general, and those derived from the olive and wine industries in particular, constitute an exciting alternative despite being underexplored for the control of citrus canker.
The agro-industrial processing of olive (Olea europaea) and grape (Vitis vinifera) generate large amounts of agro-industrial by-products, such as the semisolid residue from the extraction of olive oil (hereafter mentioned as ‘Alperujo’), which is composed by olive skins (epicarp), the pulp (mesocarp), and stones (endocarp) (Nunes et al., 2016; Pasten et al., 2019) and grape (hereafter mentioned as ‘Orujo’) obtained after the extraction of grape juice during the wine-making process (Baraybar and Monje, 2017; Varoa-Suarez et al., 2017; Silva et al., 2021). Such sub-products are rich in bioactive phenolic compounds, including trans-resveratrol and other stilbenes, quercetin, proanthocyanidins, anthocyanins, hydroxybenzoic and hydroxycinnamic acids, and catechins in the case of grape pomace (Peixoto et al., 2018; Silva et al., 2021; Averilla et al. 2019; Difonzo et al., 2021) and luteolin, vanillic acid hexoside, oleuropein, quinic acid, and 2-hydroxy oleuropein in Alperujo (Viola et al., 2022). The Alperujo and Orujo extracts could successfully be applied for crop protection, particularly for citrus canker control. Therefore, this study aims, on the one hand, to characterize X. citri strains isolated from Tucumán, Argentina, and on the other, to evaluate the potential of olive and wine agro-industrial wastes as alternatives for the biological control of citrus canker.